Implantable blood pumps may be used to provide assistance to patients with late stage heart disease. Blood pumps operate by receiving blood from a patient's vascular system and impelling the blood back into the patient's vascular system. By adding momentum and pressure to the patient's blood flow, blood pumps may augment or replace the pumping action of the heart. For example, a blood pump may be configured as a ventricular assist device or “VAD.” Where a VAD is used to assist the pumping action of the left ventricle, the device draws blood from the left ventricle of the heart and discharges the blood into the aorta.
To provide clinically useful assistance to the heart, blood pumps impel blood at a substantial blood flow rate. For an adult human patient, a ventricular assist device may be arranged to pump blood at about 1-10 liters per minute at a differential pressure across the pump of about 10-110 mm Hg, depending on the needs of the patient. The needs of the patient may vary with age, height, and other factors.
If a VAD is operated at an average flow rate in excess of the average inflow rate of blood to the ventricle, the VAD will create a suction condition within the ventricle, wherein the ventricle is collapsed and essentially devoid of blood. This condition is undesirable. For example, in such a condition the wall of the ventricle may collapse in such a way that the wall occludes the pump inlet, causing the flow rate through the pump to decline rapidly, leading to inadequate blood perfusion. Moreover, if a suction condition is maintained for a prolonged period, it can cause damage to the heart. Accordingly, as disclosed, for example, in U.S. Patent Application Publication Nos. 2015/0367048 (“the '048 Publication”) and 2014/0100413 (“the '413 Publication”), the disclosures of which are incorporated by reference herein, VAD control systems have been arranged to detect suction conditions. For example, the '048 Publication discloses methods in which the control system associated with the pump acquires data representing flow rate through the pump and examines this data to detect a suction condition. In one embodiment, the control system examines the minimum flow rate occurring during one or more cardiac cycles, the amplitude of the flow rate waveform and the average flow rate to yield a calculated value. The control system may examine a plurality of these calculated values representing several cardiac cycles and determine properties such as the mean, median, mode and standard deviation of such values to determine whether or not a suction condition exists. As disclosed in the '413 Publication, the control system may compare a minimum flow rate occurring during a current cardiac cycle against a threshold which is set based on minima of previous cardiac cycles, and determine that a suction condition exists if the minimum flowrate for the current cardiac cycle is below the threshold. In either case, the control system may respond to detection of a suction condition by altering operation of the pump as, for example, by reducing the set rotational speed of the rotor, in an effort to clear the suction condition, by issuing an alarm signal, or both. Despite these and other improvements in the art, still further improvements would be desirable. For example, systems based on thresholds established during previous cardiac cycles can be susceptible to false alarms when the set rotational speed of the rotor is deliberately changed. In some cases, the suction detection function is disabled during speed changes, during pump startup or both. Moreover, if a suction condition exists immediately after startup of the VAD or when the detection system is re-enabled after being disabled, the thresholds may be set based on conditions prevailing during the suction condition, and the system may ignore the suction condition.